Prodrug anti-cancer therapy

ABSTRACT

Compositions and methods for inhibiting the growth of cancer cells are provided. The cancer cells, the growth of which is inhibited, have constitutively active Abl tyrosine kinase activity due to a t(9;22)(q34;q11) translocation which results in expression of a chimeric Bcr-Abl protein which has constitutively active Abl tyrosine kinase activity that is believed to play an important role in leukemogenesis. The compositions include a modified protein kinase C (PKC) which has an Abl tyrosine kinase target motif. The methods involve administering the modified PCK to an individual to inhibit the growth of cancer cells that have Abl tyrosine kinase activity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser. No. 12/820,827, filed Jun. 22, 2010, and claims priority to U.S. provisional application No. 61/219,220, filed Jun. 22, 2009, the disclosures of each of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to cancer therapy and more particularly to the design and use of prodrugs to inhibit the growth of cancer cells.

DISCUSSION OF RELATED ART

Cancer continues to be a significant cause of death in humans throughout the world. The search for cancer therapies is complicated by the focus of pharmaceutical interventions on inhibition of signal transduction pathways that are involved in the pro-survival/anti-differentiation aspect of cancer cells. A prototypical example of such a strategy is inhibition of the activity of the fusion oncogene Bcr-Abl, which is expressed in both chronic myelogenous leukemia (CLL) and acute lympohocytic leukemia (ALL). Bcr-Abl has unregulated kinase activity, which is essential form all CML basts to survive and proliferate. The kinase inhibitor sold under the brand GLEEVAC (imatinib) is an Abl-specific inhibitor and is now the standard of care for all phases of CML. Further, imatinib has become the paradigm for inhibiting oncogenic signal transduction as a means of altering leukemic blast (and other transformed cell) biology. However, studies of imatinib-resistant CML have shown that amplification of the bcr-abl gene or mutations in the kinase domain of Bcr-Abl are major mechanisms of drug resistance—demonstrating that CML blasts do not become Bcr-Abl “independent” and underscoring the importance of continued oncogenic signaling for leukemia survival. However, many other cancers, including Bcr-Abl+ ALL, have abnormal oncogenic signaling, yet inhibiting this pathway does not necessarily affect the cancer cells because they do not need the signaling pathway for survival. Thus, there is an ongoing need for therapeutic cancer interventions that can exploit endogenous cancer cell specific genes without reliance on inhibiting the function of such genes.

SUMMARY OF THE INVENTION

The present invention provides compositions and methods for inhibiting the growth of cancer cells in an individual. The invention is particularly suited for inhibiting the growth of cancer cells that have constitutively active Abl tyrosine kinase activity, such as Chronic myeloid leukemia (CML) cells, which harbor a:t(9;22)(q34;q11) translocation. This chromosomal rearrangement results in a bcr-abl fusion gene. A chimeric Bcr-Abl protein which has constitutively active Abl tyrosine kinase activity that is believed to play an important role in leukemogenesis is expressed from the bcr-abl fusion gene. Thus, in one embodiment, the invention provides a method for inhibiting growth of cancer cells in an individual, wherein the cancer cells have constitutively active Abl tyrosine kinase activity. The method comprises administering to the individual a modified protein kinase C (PKC) which comprises an Abl tyrosine kinase target motif. It is believed that the modified PKC is phosphorylated by a Bcr-Abl protein expressed by the cancer cells, which results in inhibition of growth of the cells.

The modified PKC is in one embodiment human PKC βII which has been modified to express an Abl tyrosine kinase target motif that comprises the sequence of SEQ ID NO:6. The modified PKC βII can also comprise a cellular transduction domain, such as the HIV-TAT cellular transduction domain. In one embodiment, the cellular transduction domain comprises SEQ ID NO:7.

The method of the invention is particularly suited for providing a therapeutic benefit to an individual who is diagnosed with, suspected of having, or at risk for developing leukemia. In one embodiment, the leukemia is CML. In another embodiment, the leukemia is acute lympohocytic leukemia (ALL).

The invention also comprises a composition comprising a modified protein kinase C (PKC) βII, wherein the modified PCKβII comprises an Abl tyrosine kinase target motif. In one embodiment, the Abl tyrosine kinase target motif comprises the sequence of SEQ ID NO:6. As with the method of the invention, the composition can comprise a modified PKC βII that contains a cellular transduction domain. In one embodiment, the cellular transduction domain comprises SEQ ID NO:7.

The invention also includes a modified PKCβII that is present in a cancer cell which has constitutively active Abl tyrosine kinase activity, and wherein the modified PKCβII is phosphorylated on a Tyr present in the Abl tyrosine kinase target motif.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A presents a diagrammatic representation of PKC βII showing A25Y mutation in the pseudosubstrate domain. (Spitaler & Cantrell, Nat. Immunol. 2004) FIG. 1B presents partial amino acid sequence of the wild type PKC bII and A250Y mutants. Sequences depicted in FIG. 1B are: WT (SEQ ID NO:1); A1-2 (SEQ ID NO:2); A2-1 (SEQ ID NO:3); A3-4 (SEQ ID NO:5); A4-6 (SEQ ID NO:5).

FIG. 2 presents a confocal miscroscopy analysis wherein the wild type (WT) or A25Y A1-2 PKC βII-GFP clone were transiently transfected into K562 (Bcr-Abl+) or KG1a (Bcr-Abl−) and after 24 hr visualized by confocal microscopy for the GFP tag after no treatment (media) or PMA treatment for 30 minutes.

FIG. 3 provides a photographic representation of an electrophoretic analysis of the expression of the wild type and A25Y PKC βII-GFP protein in K562. K562 cells were transiently transfected with GFP only, wild type PKC-βII-GFP, or the mutated A25Y constructs. Transfections were performed by Amaxa nucleofection, with the transfection efficiency for the constructs ranging from 83-90%. At the timepoints indicated, cells were lysed and equal amounts of protein were analyzed for PKC βII expression by Western blot.

FIG. 4 provides a photographic representation of electrophoretic analysis of tyrosine phosphorylation of the A25Y PKC βII-GFP clones. K562 cells were transfected with GFP, WT-PKC βII-GFP or A25Y PKC βII-GFP clones and 24 h later, cell extracts were immunoprecipitated (IP) with anti-PKC βII antibody followed by immunoblotting (IB) with anti-phospho-tyrosine (upper panel) The blot was stripped and reprobed with anti-PKC βII to confirm equal loading (lower panel).

FIG. 5 provides a graphical representation of data obtained from analysis of cell expansion. K562 cells were transfected with GFP alone, wild type PKC bII-GFP or the mutated A25Y constructs. Three million cells were plated in culture, and viable cell numbers counted after 7 days.

FIG. 6 provides a graphical representation of data obtained from analysis of tumor growth in mice. 10×10⁶ K562 cells were inoculated SQ in NOD/SCID mice and tumor growth was monitored at indicated time points.

FIG. 7 provides a fluorescent microscopy analysis of pTAT cDNA clones nucleogected in Bcr-Abl+ K562 cells. After 24 hours cells were imaged by fluorescent microscopy in media alone, or 1 hour after the addition of the PCT agonist PMA.

FIG. 8 provides a photographic representation of analysis of bacterial lysates from bacteria (BL21) transformed to express recombinant proteins of the invention. Proteins from cells uninduced (−) or induced with IPTG (+) were analyzed on an 8% gele and stained for proteins. The arrow points to induced proteins of the expected size.

FIG. 9 provides a photographic representation of fluorescent microscopy analysis of bacterial protein lysates containing TAT-A25Y-PKC βII-GFP transduced into K652 cells (20 h).

FIG. 10 a photographic representation of confocal microscopy imaging of WT and A1-2 PKCBII-GFP clones 24 hours after transfection into K562 (Bcr/Abl+) and KG1a (Bcr/Abl−) cells. Cells were visualized after no treatment (media) or 30 minutes after PMA treatment.

FIGS. 11A and 11B provide graphical representations of data obtained from flow cytometric analysis performed on WT and A25Y transfected K562 cells. Transfected cells were counted and harvested at the indicated time points and stained with AnnexinV-PE for 30 minutes, then measured for expression of GFP (A) and AnnexinV (B) using flow cytometry. AnnexinV+ cells were gated on the GFP+ population.

FIG. 12 provides photographic representations of microscopy imaging of both WT and A1-2 TAT-A25Y-PKCBII Amaxa transfected, and bacterial lysate added K562 cells. Bacterial lysates were purified for the TAT-A25Y protein and then added onto K562 cells for 20 hours.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides compositions and methods for inhibiting the growth of cancer cells in an individual. The method comprises administering to an individual in need of treatment a pro-drug, wherein the pro-drug is a substrate for an enzyme, where the enzyme is endogenous to the cancer cells, and wherein the enzyme is preferentially or exclusively expressed by the cancer cells relative to non-cancer cells. The enzyme that is preferentially or exclusively expressed by the cancer cells relative to non-cancer cells is accordingly considered to be a cancer cell specific enzyme. Conversion of the pro-drug to the active drug by the cancer cell specific enzyme results in inhibition of the growth of the cancer cells. It will be recognized therefore that the present invention is distinct from known methods of gene directed enzyme prodrug therapy (GDEPT) which seek to experimentally alter cancer cells so as to express an exogenous enzyme that activates a pro-drug in the cancer cells.

In various embodiments, the cancer cells inhibited by the method of the present invention may be cancer cells present in solid tumors, or cancer cells present in blood, such as leukemia cells.

In one embodiment, the cancer cell specific enzyme is the Bcr-Abl protein, which exhibits constitutively active Abl tyrosine kinase activity and is expressed in cancer cells comprising a reciprocal chromosomal translocation t(9;22)(q34;q11). The amino acid sequence of the Bcr-Abl protein is known to those skilled in the art, and the skilled artisan can readily determine whether or not a particular cell type obtained from an individual displays constitutively active Abl tyrosine kinase activity. Thus, in one embodiment, the method entails inhibiting growth of cancer cells in an individual, wherein the cancer cells express constitutively active Abl tyrosine kinase activity. In one embodiment, expression of Abl tyrosine kinase is necessary for survival of the cancer cells.

The reciprocal chromosomal translocation known to those skilled in the art by the conventional designation: t(9;22)(q34;q11) is found in, for example, Chronic myeloid leukemia (CML) cells, and the hallmark of CML is the bcr-abl fusion gene, which arises from this reciprocal chromosomal translocation t(9;22)(q34;q11) and results in expression of the chimeric Bcr-Abl protein. The protein is constitutively active in and is believed to be responsible for leukemogenesis.

It is notable that normal (or wild type) PKC is not a substrate for, nor is it directly activated by, tyrosine kinases. However, we demonstrate in the present invention that engineering the Abl kinase target motif into the PKC βII molecule allows the catalytic activity of Bcr-Abl to cross-activate the PKC βII signaling pathway, resulting in direct induction of PKC-mediated cellular responses (differentiation/growth arrest/apoptosis).

Our results indicate that our modified PKC βII molecules are activated in the Bcr-Abl⁺ CML line K562, but not in the Bcr-Abl⁻ AML cell line KG1a. Our modified proteins also undergo tyrosine phosphorylation and inhibit the expansion of CML blast numbers. Thus the invention is expected to be particularly useful for therapy of malignancies such as CML, where resistance to tyrosine kinase inhibitors is due (in part) to an amplified signal—or in other malignancies where the oncogenic signaling pathway is on but blocking it has no effect (i.e. epidermal growth factor receptor signaling). Further, in contrast to signaling inhibitors, the strategy employed by the present invention utilizes aberrant oncogenic signaling to activate prodrug effector molecules, which can facilitate a cascade effect of a small level of activation at the top of the pathway into a large downstream effect. Also, in contrast to kinase inhibitors, resistance mechanisms that amplify the targeted signaling pathway would actually enhance the efficacy of rewired signaling molecules. Finally, the method of the invention is not dependent on whether the targeted signaling pathway is important for the survival of the malignant cell (unlike small molecule inhibitors), but only that it is on. We also demonstrate that use of a recombinant PKC protein comprising the Abl phosphorylation site, when fused to a cellular transduction domain, is capable of entering cells, thus supporting use of the protein in therapeutic applications.

In a specific embodiment, the pro-drug is a protein kinase C (PKC) Amino acid sequences of human PKCs are well known in the art. While any PKC is expected to be suitable for use in the invention, in one embodiment, the PKC is a PKC βII isoform. An exemplary wild type human PKC βII isoform sequence is provided in SEQ ID NO:8.

A wild type PKC is modified according to the invention so as to comprise the Abl kinase target motif (Ala-X-X-Ile-Tyr-X-X-Phe/Pro (SEQ ID NO:6). Without wishing to be bound by theory, it is believed that the modified PKC, such as a modified human PKC βII, can exploit the catalytic activity of Bcr-Abl in, for instance, human CML cells, to phosphorylate and thereby activate the PKC βII signaling pathway, resulting in induction of PKC-mediated cellular responses (which can include but are not necessarily limited to differentiation/growth arrest/apoptosis) of the cells. The invention therefore provides in one embodiment a method for inhibiting growth of CML cells in an individual comprising administering to the individual a modified PKC βII protein, wherein the protein comprises an Abl tyrosine kinase target motif.

In one embodiment, the invention provides a composition comprising the pro-drug, wherein the pro-drug is a modified PKC βII protein comprising an Abl tyrosine kinase target motif. Also provided are polynucleotides encoding modified PKC βII proteins. Those skilled in the art will recognize that the compositions of the invention are useful not only for therapeutic purposes as set forth in the description of the method of the invention, but also for studying the metabolic and other characteristics of cancer cells, and in particular leukemia cells that express an Abl tyrosine kinase, where that expression at least in part distinguishes the cancer cells from other non-malignant cells.

In addition to the Abl kinase target motif, various other modifications may be made to the PKCs provided by the invention.

In one embodiment, the invention provides the pro-drug as a recombinant protein, wherein the protein comprises a protein transduction domain. The transduction domain can be one which enhances the capability of the protein to enter cells. In one embodiment, the transduction domain is the HIV-TAT domain. Those skilled in the art will recognize that “TAT” represents “Trans-Activator of Transcription”, and that HIV-TAT proteins generally have a length of between 86 and 101 amino acids, dependent upon which HIV subtype encodes the particular TAT protein in question. However, the TAT proteins have in common a cellular transduction domain having the sequence YGRKKRRQRRR (SEQ ID NO:7). Thus, in one embodiment, the modified proteins of the invention comprise a cellular transduction domain that comprises or consists of SEQ ID NO:7. Non-limiting examples of sequences that can comprise the TAT domain are shown in SEQ ID NOs:15, 16, 17 and 18, wherein the TAT domain is present at amino acid positions 39-50 in each sequence, but wherein the amino acids at positions 1-38 may also include TAT protein sequences, or other sequences, such as sequences for use in isolating and/or purifying the protein. Therefore, the present invention includes but is not limited to compositions and methods which include proteins comprising the amino acid sequences depicted in SEQ ID NOs:15, 16, 17 and 18, with the proviso that the inclusion of amino acid sequences in positions 1-38 of these sequences is optional. For instance, HIS tag sequences, such as the His shown at positions 5-10 in each of these sequences, are optional (as is the entire amino acid sequence, and any portion thereof, from position 1-38 in each of these sequences). Thus, the invention provides compositions and methods that involve proteins that may begin at their N-terminus at position 39 in SEQ ID NOs:15, 16, 17 and 18.

The proteins provided by the invention also optionally comprise amino acid sequences that constitute reporter proteins. Many reporter genes encoding reporter proteins are known to those skilled in the art and are suitable for use in the invention. In one embodiment, the proteins of the invention comprise a green fluorescent protein (GFP) sequence. For instance, in SEQ ID NOs:15, 16, 17 and 18, the amino acids in positions 757 through the C-terminus comprise a GFP sequence. As such, they represent optional amino acid sequences that may or may not be present in the proteins provided by the invention. Thus, in various embodiments, a protein of the invention comprises an amino acid sequence shown in SEQ ID NOs:15, 16, 17 and 18, with the proviso that the amino acids 1-38 and 757 through the C-terminus are optional and may or may not be present in the proteins. The protein of the invention may therefore comprise or consist of an amino acid sequence having its N-terminus at amino acid position 39 in SEQ ID NOs:15, 16, 17 or 18, and may further comprise the contiguous amino acids of SEQ ID NOs:15, 16, 17 or 18 from position 39 up to amino acid 756, inclusive, in said sequences. Further, such proteins may comprise or consist of an amino acid sequences having its N-terminus at amino acid position 1-39, inclusive, in SEQ ID NOs:15, 16, 17 or 18, which proteins may have at their C-termini amino acid 756 in said proteins, or which may comprise or consist of the entire amino acid sequence shown in SEQ ID NOs:15, 16, 17 or 18.

It will be recognized that the modified proteins provided by the invention as described supra also comprise the Abl kinase target motif (Ala-X-X-Ile-Tyr-X-X-Phe/Pro (SEQ ID NO:6). In various embodiments, the Abl kinase target motif can be present starting at position 21 in the PKC βII protein, such as is shown in the SEQ ID NOs: 10-13 and 14-18, although those skilled in the art that the location of the Abl kinase target motif can be positioned in other places in the protein, so long as the protein remains being capable of activation when phosphorylated by the Abl kinase. In various embodiments, the Abl kinase target motif can be any of SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4 or SEQ ID NO:5.

In one embodiment, the invention provides a modified PKCβII, wherein the modified PKCβII is present in a leukemia cell, such as a CML cell, wherein the modified PKCβII is phosphorylated on a tyrosine present in the Abl tyrosine kinase target motif. In one embodiment, the phosphorylated Tyr is the fifth amino acid in the Abl tyrosine kinase target motif.

Compositions comprising the pro-drug may be prepared by mixing the pro-drug with any suitable pharmaceutically acceptable carriers, excipients and/or stabilizers. Some examples of compositions suitable for mixing with the pro-drug can be found in: Remington: The Science and Practice of Pharmacy (2005) 21st Edition, Philadelphia, Pa. Lippincott Williams & Wilkins.

It is expected that the pro-drug can be delivered to an individual in need of treatment using any available method and route suitable for pro-drug delivery, including oral, parenteral, subcutaneous, intraperitoneal, intrapulmonary, and intranasal. Parenteral infusions include intramuscular, intravenous, intraarterial, intraperitoneal, and subcutaneous administration. Those skilled in the art, given the benefit of the present disclosure, will be able to determine an effective amount of the modified protein to administer to the individual based on such factors as the age, sex, and size of the individual and the state of the disease.

In one embodiment, a composition comprising the pro-drug is administered to an individual who is diagnosed with, suspected of having, or at risk for developing leukemia. In one embodiment, the leukemia is Chronic myeloid leukemia (CML).

Administration of the pro-drug can be performed in conjunction with conventional therapies that are intended to treat a disease or disorder associated with activated Abl kinase. For example, the pro-drug could be administered prior to, concurrently, or subsequent to conventional anti-cancer therapies. Such therapies can include but are not limited to chemotherapies, surgical interventions, and radiation therapy.

The following Examples are meant to illustrate but not limit the present invention.

EXAMPLE 1

This Example illustrates one embodiment of the present invention, whereby growth of CML cells is inhibited via administration of a pro-drug to the cells in the form of a genetically modified PKC βII comprising the Abl kinase target motif (Ala-X-X-Ile-Tyr-X-X-Phe/Pro), wherein the CML specific enzyme Bcr-Abl phosphorylates the PKC βII pro-drug to convert it to an activated drug form, thereby causing PKC-mediated inhibition of the growth of the CML cells.

Construction of A25Y PKC βII mutants. Substitution of Ala²⁵ with the phosphomimetic residue Glu results in a constitutively active PKC. We tested whether mutation of Ala²⁵ in the auto-inhibitory pseudosubstrate domain of PKC βII to a phosphorylatable tyrosine (A25Y) within an Abl target motif would, upon direct Bcr-Abl phosphorylation, introduce a negative charge and cause disassociation of the pseudosubstrate domain from the catalytic site, resulting in PKC βII activation (FIG. 1A). This Bcr-Abl-activated PKC βII signaling should then induce cellular responses, potentially including blast differentiation/growth arrest/apoptosis. Kharfan-Dabaja, M. A., Ayala, E., Guller, I., Cejas, P. J., Bahlis, N. J., Kolonias, D., Carlson, L. M., and Lee, K. P. Differentiation of acute and chronic myeloid leukemia into functional dendritic cells: a comparative analysis of different signaling mechanisms. Cancer Immunol Immunother. 2005, 54 (1): p. 25-36 Lindner I, Kharfan-Dabaja M A, Ayala E, Kolonias D, Carlson L M, Beazer-Barclay Y, Scherf U, Hnatyszyn J H, Lee K P. Induced dendritic cell differentiation of chronic myeloid leukemia blasts is associated with down-regulation of BCR-ABL. J. Immunol. 2003 Aug. 15; 171(4):1780-91.

The native PKC βII core pseudosubstrate sequence is Arg-Phe-Ala-Arg-Lys-Gly-Ala-Lys-Arg-Gln-Lys-Asn-Val (WT, FIG. 1B; sequence identifiers are as shown in FIG. 1B). We used oligonucleotide primers/PCR to convert the underlined sequence to the following Bcr-Abl target motifs (changes from the native sequence in bold italics, tyrosine is the phosphorylated residue): clone A1-2 (Ala-Arg-Lys-Ile-Tyr-Lys-Arg-Pro), clone A2-1 (Ala-Arg-Lys-Ile-Tyr-Lys-Arg-Phe (Phe more closely resembles the Gln it is replacing than does Pro)), clone A3-4 (Ala-Arg-Lys-Ile-Tyr-Lys-Arg-Thr (Thr being a polar/uncharged residue like Gln it replaces)) and clone A4-6 (Ala-Arg-Lys-Ser-Tyr-Lys-Arg-Phe (Ser being a polar/uncharged residue like the Gly it replaces. We have made these A25 Y (A25Y) mutants in PKC-βII-GFP, which is PKC βII genetically fused to GFP (which does not affect its PKC activity). The GFP tag allows for direct visualization of enzyme activation by membrane translocation, and also is bigger than the endogenous enzyme as determined by Western blot.

Selective activation of A25Y PKC βII-GFP in the Bcr-Abl-positive CML cell line K562. Activation of PKC βII results in translocation of the enzyme from the cytosol to the plasma membrane, and this can be tracked in the wild type (WT) or A25Y PKC βII-GFP mutants by confocal microscopy imaging of the GFP tag. These constructs were nucleofected (Amaxa) into the Bcr-Abl-positive CML cell line K562 or the Bcr-Abl-negative AML cell line KG1a, and transfectants left untreated (media) or PMA treated. As seen in FIG. 2, the parental PKC βII-GFP does not translocate in either K562 or KG1a unless stimulated by the PKC agonist PMA. This indicates that if there is any indirect activation of PKC βII by Bcr-Abl in the untreated K562, it is happening at a level too low to detect by this approach. In contrast, the A2-1 A25Y PKC βII-GFP mutant is translocated to the plasma membrane in K562 cells in the media-only condition, and further activated by PMA addition (indicating the mutated PKC is still responsive to PKC activation stimuli). However, it is not activated in Bcr-Abl-negative KG1a cells unless PMA is added. By confocal imaging we have found that three constructs are activated in K562 but not KG1a (two clearly (clones A1-2, A-3-4) and 1 less strongly (A2-1)), while the fourth (A4-6) appears to be constitutively active. These findings indicates that 3 of the A25Y PKC βII-GFP constructs are being activated by Bcr-Abl.

Evidence of activation-induced PKC degradation. We examined whether activation of PKC βII-GFP protein results in its degradation and loss, a general characteristic of the PKC family, in the transfected K562 in media alone, with the prediction that ongoing Bcr-Abl activation of the A25Y PKC βII-GFP may cause more rapid loss of these proteins vs. WT PKC βII-GFP. It should be noted that the transfection efficiency of K562 cells by Amaxa nucleofection was uniformly high (83-90%) for all the constructs. The expression of wild type PKC βII-GFP and A25Y PKC βII-GFP (which are larger than the endogenous PKC βII) over time in K562 cells was assessed by Western blot (FIG. 3). 24 hours after transfection there are equivalent protein levels of WT PKC-βII-GFP vs. the A25Y PKC βII-GFP mutants (see FIG. 4, bottom panel). Also, at a 16 hr time point there was a similar amount of GFP expression (by flow cytometry) between WT and mutant A25Y PKCβII transfectants (data not shown). However, by day 3 there is substantially less A25Y vs. WT PKC βII-GFP. And although there is some loss of WT PKC βII-GFP expression over time, the expression of the A25Y PKC βII-GFP protein is more rapidly lost—for example, A1-2 expression on day 3 vs. day 5 compared to WT βII expression on day 5 (which is similar to A1-2 expression on day 3) vs. day 7. This differential effect makes it unlikely this is due to non-specific loss of the plasmids, and two non-mutually exclusive possibilities for accelerated loss of A25Y PKCβII expression are increased PKC activation (by Bcr-Abl activation) and/or growth suppression/killing of the transfected cells.

Tyrosine phosphorylation of A25Y PKC βII-GFP mutants. Plasma membrane translocation and loss of protein expression suggest that the A25Y PKC βII-GFP mutants are being activated in K562, but do not indicate how. In addition to Bcr-Abl-mediated activation, it is formally possible that they are constitutively active (which would result from any significant structural disruption of the pseudosubstrate domain, and as we suspect for the A4-6 construct), have a lower activation threshold for the traditional PKC second messengers, or are being activated via some other signaling pathway. If the A25Y PKC βII mutants are being activated by Bcr-Abl, the engineered tyrosine should be phosphorylated. To assess this, K562 cells were transfected with GFP, WT-PKC βII-GFP and clones A1-2 (which clearly translocates in K562 (in media) by confocal) and A2-1 (which translocates less well). 24 h later, cell extracts were immunoprecipitated with anti-PKC βII antibody, and then phosphorylation was assessed by western blot analysis with phospho-tyrosine antibody. As seen in FIG. 4 (top panel), there is no evidence for tyrosine phosphorylation of endogenous PKC βII (for example, in the control and GFP transfection lanes), of WT PKC βII-GFP (also indicating that tyrosine residues in the GFP tag is not being phosphorylated), or (somewhat unexpectedly) of the A2-1 clone. The A1-2 clone is clearly tyrosine phosphorylated—and given this clone differs from the WT PKC-βII-GFP in tyrosine content only at the engineered Abl-target motif, it is likely that it is the Tyr²⁵ that is being phosphorylated. The bottom panel (FIG. 4) demonstrates that similar amounts of PKC βII-GFP were immunoprecipitated in all 3 PKC transfectants (the IgG_(H) band is from the original IP, identified on Western blot by the secondary antibody).

Transfection of A25Y PKC βII mutants reduces the growth of K562 cells. To assess the effect of A25Y PKC βII-GFP on cell expansion, equal numbers of transfected K562 were plated in media alone and counted after 7 days (FIG. 5). There is some inhibition of cell expansion by the WT PKC βII-GFP vs. GFP-only transfected K562, with substantially greater inhibition by the A25Y PKC βII-constructs. Thus, this Example demonstrates inhibiting growth of cancer cells comprising administering to the cells a pro-drug, wherein the pro-drug is a substrate for an enzyme, wherein the enzyme is endogenous to the cancer cells, and wherein the enzyme is exclusively expressed by the cancer cells relative to non-cancer cells, wherein the pro-drug is converted to a drug by the enzyme, and wherein the growth of the cells is inhibited by the drug subsequent to conversion of the pro-drug to the drug.

EXAMPLE 2 pTAT-A25Y PKC βII-GFP Construct

This Example demonstrates development of pTAT-A25Y PKC βII-GFP construct for use in delivering the prodrug to cells.

In order to deliver the A25Y PKC βII-GFP proteins to cells and in vivo, we cloned the cDNA constructs into the pTAT-HA vector, which adds the HIV TAT transducer domain (plus the HA tag) to the N terminus of the A25Y PKC βII-GFP. The incorporated HIV TAT transducer domain has been shown to allow large proteins to readily cross cell membranes and diffuse intracellularly in vitro and in vivo. This vector can be transformed into bacteria and production of the encoded protein induced by the addition of IPTG. This construct also introduces 6 histadines (His6) into the protein that allows for purification of the protein over a nickel bead column, as well as an HA tag that allows for the Western blot analysis for this epitope. The pTAT cector was digested with XhoI and filled in to blunt the 3′ cloning end, the 5′ end was digested with KpnI and the A25Y PKC βII-EGFP cDNA insert was double digested with KpnI and NaeI and ligated into the pTAT vector digested with KpnI and blunted at XhoI site.

In addition, the TAT-A25Y-PKC βII-GFP insert was subcloned into the eukaryotic expression vector pcDNA3, which allows us to transfect the construct into cells and determine if it remains active despite the addition of the TAT-HA elements to the protein (see below).

Functional Assessment of TAT-A25Y PKC βII-GFP.

Because the addition of the TAT-HA domains to the N terminus of the A25Y-PKC βII-GFP proteins has the potential to disrupt the protein's structure adjacent to the pseudosubstrate domain, it is possible that this addition results in a constitutively active protein that no longer requires activation by PKC agonists (PMA in our experiments) or Bcr-Abl. Conversely, these modifications may prevent activation altogether. To test this, we transfected the TAT-A25Y-PKC βII-GFP constructs in the pcDNA3 vector into the Bcr-Abl+ CML cell line K562. K562 cells were split to 0.5×106 cells/ml 20-24 hours before transfection. 2×106 cells per sample to be transfected were spun down at 850 RPM for 10 min. The cells were then resuspended in 100 μl of Amaxa Reagent V plus supplement per sample and mixed with 5-10 μg of DNA. Cells were then transfected using Amaxa protocol T-16 and then transferred to a 6 well plate containing 2 mls of media prewarmed to 370. After 3-4 hours of being incubated at 370 3 mls of media were added to each well and continued to be incubated for 24-48. Translocation of the PKC βII enzyme from the cytoplasm to the plasma cell member is indication that the enzyme has been activated, and with our TAT-A25Y-PKC βII-GFP proteins can be followed on the basis of the fluorescent GFP tag using fluorescent microscopy to determine subcellular localization. As seen in FIG. 7, compared to the control parental PKC βII-GFP, there appears to be no constitutive activation of TAT-PKC βII-GFP under media conditions indicating that addition of the TAT-HA did not disrupt the regulatory pseudosubstrate domain.

The two TAT-A25Y-PKC βII-GFP clones (A1-2-2 and A1-2-8) show some activation in the K562, which we believe is due to Bcr-Abl-mediated activation. For all the constructs, there is significant activation with the PKC agonist PMA, indicating that the TAT-PKC βII-GFP clones are still responsive to appropriate activation signals. Together, these data indicate that adding the TAT-HA domain does not affect the behavior of the original PKC βII-GFP constructs.

Bacterially Synthesized TAT-A25Y-PKC βII-GFP Protein.

To synthesize the TAT-A25Y-PKC βII-GFP protein for administration, the pTAT constructs were transformed into competent BL21(DES) cells and plated on LB agar+carbanicillin plates and grown overnight at 370. A 5 ml culture (LB+ carb) was inoculated with a single colony of BL21 (DES) transformed cells and grown overnight at 370. The overnight culture was used to inoculate 50 mls of LB+ carb. Bacteria were grown to an O.D. of 0.6 and then induced with IPTG at a concentration of 1 mM and grown overnight at 370 with shaking. The cells were pelleted in 15 ml aliquots by centrifuging at 3500 RPM for 15 minutes. The cell pellets were either frozen at −800 or directly lysed. The bacterial pellet was lysed in 3 mls of lysis buffer+lysozyme added at 1 mg/ml and incubated on ice for 30 minutes. Lysates were then sonicated for 6 cycles of 10 seconds—sonication/10 seconds on ice. Proteins were concentrated using a Viva Spin column (30,000 MW cut off) with 3 washes of PBS. The protein lysates were then analyzed by on a Comassie gel (FIG. 8), which demonstrates bacterial synthesis of the proteins. To determine if the proteins can be transduced into live cells, K562 cells were plated at 0.1×106 cells in 0.5 mls of 10% serum media in a 24 well plate. ˜500 μg of total concentrated bacterial induced protein (not additionally purified at this point) for each construct were added to each well. The cells were then incubated overnight at 370. Transduction of the bacterially synthesized TAT-A25Y-PKC βII-GFP protein was then examined by fluorescent microscopy for the GFP tag. As seen in FIG. 9, these proteins are capable of being transduced into cells, even as crude lysates that have not been further purified. These data indicate that we can synthesize recombinant TAT-A25Y-PKC βII-GFP protein that retains the characteristics of the parental A25Y-PKC βII-GFP molecules, and can be delivered to cells via TAT transduction.

EXAMPLE 3

This Example demonstrates that Mutated PKCBII translocates to the plasma membrane in Bcr/Abl+ K562 but not Bcr/Abl− KG1a cells.

Upon activation, PKCBII translocates from the cytoplasm to the plasma membrane. To determine the activation status of PKCBII through translocation to the plasma membrane, WT and A25Y PKCBII-GFP mutants were Amaxa nucleofected into K562(Bcr/Abl+) and KG1a (Bcr/Abl−), and then visualized using confocal microscopy either after no treatment in media, or 30 minutes after PKCBII agonist PMA treatment. As seen in FIG. 10, the WT PKCBII-GFP construct is cytosolically localized in media treated Bcr/Abl+, K562 cells; upon addition of PMA however, the enzyme translocates and is primarily localized to the plasma membrane. A similar trend is observed in Bcr/Abl− KG1a cells, where prior to PMA addition WT PKCBII-GFP is localized to the cytosol and only after PMA treatment does it translocate to the plasma membrane. In contrast, the A1-2 A25Y PKCBII-GFP mutant is found primarily at the plasma membrane in media treated K562 cells and is further activated by the addition of PMA, indicating that the A25Y PKCBII-GFP mutants are still responsive to traditional PKC stimuli. In Bcr/Abl− KG1a cells however, the A1-2 A25Y PKCBII-GFP mutant is not activated in media alone, and translocates to the plasma membrane only upon addition of PMA. Analysis of the 3 other A25Y mutants (A2-1, A3-4, and A4-6) revealed that while A2-1 and A3-4 are also activated in K562 but not in KG1a cells, A4-6 might be constitutively active as it is found localized to the plasma membrane in both cell types. These data suggest that at least three of the A25Y mutant constructs are activated in a Bcr/Abl dependent manner.

EXAMPLE 4

This Example demonstrates that transfection of A25Y PKCBII mutants induces apoptosis, and decreases the growth of K562 cells.

We have demonstrated that activation of PKCBII by PMA induces cellular arrest as well as increases in apoptosis in K562 cells. To determine if activation of A25Y-PKCBII mutants had comparable effects to endogenous PKCBII activation, the four A25Y-PKCBII mutants along with WT PKCBII-GFP were nucleofected (Amaxa) into K562 cells and analyzed for their levels of GFP expression. As seen in FIG. 11A, the number of GFP+ K562 cells that were transfected with WT-PKCBII steadily increases over the course of the experiment, while the number of GFP+ K562 cells that were transfected with A25Y-PKCBII mutant constructs were inhibited in their cellular expansion. To determine if activation of A25Y-PKCBII had any effects on the induction of apoptosis, K562 cells were transfected with WT and A25Y-PKCBII and measured for apoptosis using the AnnexinV-PE antibody. AnnexinV+ cells were gated on the GFP+ population. As seen in FIG. 11B, all GFP+transfected cells showed increases in the level of apoptosis at 18-28 hours after transfection, with A3-4 transfected cells showing the highest level of apoptosis. WT-PKCBII transfected cells also showed an increase in apoptosis in the 18-28 hours after transfection, this level decreased steadily over time. In contrast, the A3-4, A1-2, and A4-6 transfected cells showed higher levels of apoptosis maintained over the course of the experiment. These data suggest that upon activation by Bcr/Abl, A25Y-PKCBII induces cellular arrest and apoptosis. The greatest effect occurred in A3-4 transfected cells.

EXAMPLE 5

This Example demonstrates efficient generation and transmission of TAT-A25Y-PKCBII protein into K562 cells.

To determine whether or not the addition of the protein transduction domain (PTD) of HIV TAT onto our A25Y-PKCBII mutants would permit transduction into cells, the PTD sequence was added onto the 5′ terminus of the A25Y mutants and cloned into E. coli. Purified TAT-A25Y-PKCBII protein was then added to cultures of Bcr/Abl+ K562 cells, and visualized 20 hours later for expression of GFP using microscopy. As seen in FIG. 12, levels of GFP expression between Amaxa transfected cells and purified TAT-PKCBII added cells are equivalent. These data indicate that the TAT-A25Y-PKCBII is taken up by Bcr/Abl+ K562 cells whereupon it is to be likely activated in a similar manner as in Amaxa transfected cells.

While the above invention has been described through specific examples, routine modifications will be apparent to those skilled in the art and such modifications are intended to be within the scope of this invention. 

We claim:
 1. A method for inhibiting growth of cancer cells in an individual, wherein the cancer cells have constitutively active Abl tyrosine kinase activity, the method comprising administering to the individual a modified protein kinase C (PKC) bII, wherein the modified PCKbII comprises an Abl tyrosine kinase target motif, wherein the Abl tyrosine target motif is the eight amino acid sequence beginning at position 21 in SEQ ID NO:10, through amino acid position 28 in SEQ ID NO:10, wherein the modified PKC bII is phosphorylated by a Bcr-Abl protein expressed by the cancer cells, and wherein the growth of the cancer cells is inhibited subsequent to phosphorylation of the modified PKC bII by the Bcr-Abl protein.
 2. The method of claim 1, wherein the modified PKC bII comprises a cellular transduction domain.
 3. The method of claim 2, wherein the cellular transduction domain comprises SEQ ID NO:7.
 4. The method of claim 1, wherein the individual is an individual diagnosed with, suspected of having, or at risk for developing leukemia.
 5. The method of claim 4, wherein the leukemia is Chronic myeloid leukemia (CML). 